1. Field of the Invention
The present invention relates to a unitary structural fabric at least double biased and, more particularly, to a fabric of two or more layers with at least one of the layers being single biased. The invention also relates to the process and apparatus for making this fabric.
2. Description of the Related Art Structural fabrics have a wide variety of applications wherever high strength is required, but weight must be kept to a minimum. In particular, the aerospace, marine, and automobile industries frequently employ structural fabrics made up of many layers of structural fibers saturated with cross-linked and hardened resin as high strength materials to form composites. The layers of the composites may be biased in directions to maximize the strength of the overall product, frequently in the direction of strongest applied tension or strain.
By biased, it is intended to mean that the structural fibers of any particular layer are substantially oriented at an angle other than zero or ninety degrees to the major axes of the fabric composite (i.e. longitudinal and lateral centerlines).
One technique for forming such a fabric or composite is disclosed in U.S. Pat. No. 4,484,459 drawn to a biased multilayer structural composite stitched in a vertical direction. The fabric is made up of three layers of parallel structural fibers with at least one of the layers being biased. The layers are maintained by vertical stitching only, with no horizontal threads being present in the composite.
This type of fabric may be formed using an apparatus which consists of two or more weft lay down carriage mechanisms each aligned with a vertical stitching machine. The lay down carriage mechanisms are each oriented transversely to a device for advancing the fibers delivered therefrom into a stitching machine. At least one of the lay down carriages is oriented at an angle to the fiber advancing device and stitching machine, such that, when fibers are laid down in parallel array by each of the lay down carriages, the fibers of each are deposited on the fibers of the immediately preceding lay down carriage mechanism and are advanced into the stitching machine. The fibers from the angled lay down carriages are parallel biased with respect to the major axes of the fabric. In the stitching machine, a vertical stitch is passed between the fibers of each layer through the layers, sufficient to maintain the layers in vertical array and the fibers within each layer in parallel array. When desired, the fabric may be saturated with resin, which is subsequently cured, producing a composite.
An advantage to the '459 system is that a fabric or composite made up of two or more layers may be made with only one knitting stage. However, a disadvantage results in the complexity of the equipment used. At least one lay down carriage is necessary for each layer of fabric with each layer independently being fed into the stitching machine. Separate lay down carriages are oriented or angled with respect to the face of the stitching machine such that fibers are laid down in a parallel array, but at an angle with respect to the angle of each of the lay down carriages. Thereby, when the fibers enter the stitching machine, they too are oriented at an angle to the longitudinal center line of the fabric being formed, thus creating the biased layer. Although this process uses only one knitting stage, the use of many lay down carriages creates a machine that is complicated and costly.
A need, then, has arisen to manufacture a double biased fabric using much simpler apparatus as well as the development of an improved fabric resulting from such a process.
Another technique for producing a structural fabric and the resulting fabric is disclosed in U.S. Pat. No. 4,567,738 which relates to biased, structural fabrics to be used in reinforcing plastic shapes. The fabrics are made up of (i) structural yarn for strength and (ii) secondary yarn which holds the structural yarns parallel to each other. The secondary yarn is described as flexible and of much smaller cross-section than the structural yarn. Two separate layers of fabric are used with each having its own secondary yarn for support. Further, a complicated skewing process is used to offset the bias of at least one of the fabric layers to enhance structural integrity. However, in so doing, uniformity is lost in the resulting fabric.
The method of making the fabric in the '738 patent involves directing a first layer of structural fabric into a pair of counter-rotating rollers in contact with each other such that the longitudinal centerline of the first layer is perpendicular to the longitudinal axis of the rollers.
The first layer also comprises secondary holding fibers for maintaining the structural fibers in parallel alignment.
The first layer is led from the counter-rotating rollers into a stitching machine at an angle skewed from the original angle of orientation of the first layer. Simultaneously, a second layer of structural fabric is led into the stitching machine in a fashion such that the centerline of the second layer is perpendicular to the transverse axis of the stitching machine. The second layer of structural fabric is also comprised of a plurality of structural yarns substantially perpendicular to the centerline, and possibly also comprises structural yarns parallel to the centerline of the second layer with secondary holding fibers for maintaining the structural fibers in parallel alignment. The first and second layers then are stitched together in the stitching machine to provide a single structural fabric.
Accordingly, it is necessary to first knit one layer of fabric with secondary yarns then secondly, to knit a second layer of fabric with secondary yarns, then thirdly, to skew one layer of fabric for feed into the stitching machine, and finally, to knit the two layers together in the stitching machine. This process proves to be complicated in time and machinery, albeit with the result being a strong fabric. However, the skewing process results in distortions in the fabric and the array of yarns is not as uniform as may be desired.
Therefore, a need exists to produce a double biased fabric that is more uniform than that made by the skewing process of the '738 patent, yet requires less and simpler machinery than that required for knit fabric made by the process of the '459 patent.
Accordingly, it is one object of the present invention to provide a fabric of at least two layers of structural fabric, with the layers being biased, such that a uniform fabric is presented.
It is another object of the invention to provide a continuous process and apparatus whereby the above described fabric may be made using a noncomplicated arrangement of machinery.
These and other objects that will become apparent may be better understood by reference to the detailed description provided below.